This invention relates to a method of making a coded marking in a glass workpiece and particularly, although not exclusively, in making a machine-readable coded marking in a glass-envelope part of a CRT (cathode-ray tube).
U.S. Pat. No. 4,327,283 issued Apr. 27, 1982 to P. M. Heyman et al. describes a glass workpiece that carries a machine-readable coded marking, such as a bar-code marking, in its external surface. In one form, the glass workpiece comprises a main body, a thin undercoating on a portion of the external surface of the body, and a thin overcoating of contrasting color on the undercoating. The marking comprises a related sequence of substantially-parallel bars recessed into and through the overcoating. The marking may be made by spraying the coatings, each of which consists essentially of an aqueous suspension of pigment particles and an inorganic binder therefor, on selected areas of the workpiece and then recessing the marks into and through the overcoating. Recessing can be achieved by abrading the overcoating, as with a sandblast, or ablating the marks into the coating, as with a laser beam. The inorganic binders used in the sprayed coatings of this prior method consist essentially of a single component, usually a glass frit or an alkali silicate, such as potassium silicate.
The method and markings disclosed in the Heyman et al. patent perform well when produced at relatively slow rates upon glass-envelope parts for cathode-ray tubes. However, when the rates at which these markings are produced are accelerated so that the cycle time for drying the coatings is shortened to less than 60 seconds, many problems arise. The drying temperatures have to be raised. This leads to problems of glass breakage caused by the high temperature required for drying these coatings. Also frequency spray-gun clogging is experienced when using these prior coatings. With the shortened cycle time and higher drying temperatures, an increasing proportion of the coatings produced must be rejected for flaking and/or peeling. Also, there is a need to improve the contrast between the undercoatings and the overcoating so that the markings can be read more reliably.